Is Dilution the Solution?

Several studies published over the last few months have bolstered the case for the dilution effect in both West Nile and Lyme diseases. Washington University’s Allan looked at birds and mosquitoes along a gradient of rural-to-urban sites in the St. Louis area. As he moved from habitats with few humans to those with higher densities he found a simultaneous drop in biodiversity and rise in West Nile prevalence. “As diversity goes up, disease goes down,” says Allan, whose study was published in January 2009 in the journal Oecologica.

His findings align with work by John Swaddle, a biologist at the College of William and Mary in Virginia, published just a few months earlier in the journal PLoS ONE. Rather than counting species over tracts of land, Swaddle used public bird counts and health department reports to compare bird diversity between pairs of adjacent counties with and without human cases of West Nile. This way, he was able to factor out other potential explanations, such as differences in weather. Again, less biodiversity meant more disease.

Not all West Nile research supports such a direct relationship, however. A recent study in the Chicago area found no association between the number of species and cases of West Nile. Published in Oecologica in March 2009, it addressed the dilution effect at a finer scale than previous state or countywide analyses.

The lack of perfect consistency among the studies hints at some unanswered questions. “We have good patterns,” says A. Marm Kilpatrick, an infectious disease ecologist at the University of California, Santa Cruz. “But we don’t fully understand the mechanisms for those patterns.”

Many ecologists believe the dilution effect is more complicated than simply saying greater diversity leads to less disease. Growing evidence suggests that individual species do play a significant role. American robins, for example, appear to be one of the key hosts for West Nile. “Even if they only make up 1 to 10 percent of the birds in a site, they’re often 40 to 60 percent of the blood meals,” says Kilpatrick.

In the Lyme disease model, Ostfeld of the Cary Institute recently found that the total number of species was less predictive of disease than the actual identities and abundance of those species. The presence of white-footed mice was most vital to transmission, according to a study he co-authored in Ecology in October 2008.

This ambiguity inspired Ostfeld to continue digging deeper into disease dynamics, which is why he and his team are working in Dutchess County this year. Until the researchers are ready to begin trapping and transporting forest animals this summer, they remain busy counting creatures — and droppings.

Manipulating species, and counting feces

On this warm spring afternoon, Ostfeld’s colleague Shannon Duerr has found an unusually large number of droppings on Tract Number 36, including one circle boasting seven piles. This likely means “lots of deer, lots of mice, lots of ticks,” she says. The tally also means she can put her name on the Deer Poop Count Leader Board back at the Institute — a humorous “competition” that helps keep this messy and monotonous field work bearable.

The team’s latest research suggests that in addition to some species being unusually good hosts, others are really bad ones. Originally, a species was thought to “dilute” simply by providing a meal for ticks, but not infecting them. Now, certain animals have been found to act as especially powerful diluters because they are inhospitable, or even poisonous, hosts for ticks. “It looks like some species actually go out there and kill lots of ticks,” says Ostfeld, highlighting the opossum as one of these deadly hosts. At the other extreme, he says, white-footed mice are great for both boosting and infecting tick populations.

Just as Santa Cruz’s Kilpatrick doesn’t recommend hunting American robins — his research suggests that when robins disperse, mosquitoes’ next choice is people — Ostfeld’s work doesn’t mean that ecologists will be introducing furry “ecological traps” into urban parks anytime soon. “Imagine air-dropping opossums,” says Ostfeld. “I don’t think that would fly.”

On an experimental scale, however, Ostfeld is doing his own species manipulation in Dutchess County. His team hopes that a better understanding of the connections between habitat size, biodiversity and the spread of pathogens will eventually lead to actions that could dilute not only West Nile and Lyme disease, but also new diseases before they arise.

While people know more about animal-borne diseases now than they did several decades ago, Ostfeld sees a continuing need to alert the public to the important role biodiversity plays in keeping disease in check. “People care about their health a lot,” he says. “The notion that biodiversity might influence their probability of getting sick is a strong motivator.”

Public awareness could not only help prevent disease, Ostfeld says, it could also promote conservation — especially because the species involved are charismatic animals that are attractive to humans. “We’re talking about furry, feathered, colorful, warm fuzzy creatures,” he says. “People care about those as well.”

Related on Scienceline:

Preserving amphibian biodiversity.

What species give you Lyme disease?

Why are mosquitoes selective in who they bite?